Digital magnetic recording and readout equipment must have the capability of recording data at the prevailing data packing density and track spacing with sufficient accuracy to render the recording medium compatible with different pieces of equipment. Specifically, the recorded data must be capable of being reliably read out from the record medium using equipment other than that with which it was recorded. As packing densities have increased, greater performance demands have been placed on the equipment used for data recording as well as for data readout. For example, in present day commercially available magnetic tape recording equipment such as IBM 2400 Series tape handlers, data is recorded at densities which range between 200 and 800 bpi for NRZ-I recording in 7- and 9-track systems, while 9-track recording equipment, such as the IBM 3420 Series tape handler, is capable of recording at 1600 bpi PE and 6250 bpi GCR. As used herein, the expressions NRZ, GCR and PE designate the non-return-to-zero, group coded and phase encoded recording techniques respectively.
Consonant with the increase in the data packing densities, smaller spacings between the magnetic tracks on the recording medium have come into use as magnetic recording and readout equipment has become increasingly sophisticated. Thus, a center-to-center track spacing of 55 mil is common today for 9-track tape, while a 70 mil spacing is used with 7-track tape. For this type of recording, the spacing between adjacent tracks is normally 11 mil for a 9-track tape and 22 mil for a 7-track tape. Similar recording densities apply to disks of the type used in commercially available equipment, such as the Shugart 801 disk drive for floppy disks. Here the usual center-to-center track spacing may be 20 mil, while the spacing between adjacent tracks is 8 mil.
At such close track spacing, mistracking is a constant concern. Mistracking is present when undesired relative displacement occurs between the magnetic head and the recording medium in a direction normal to the center line of the head. Since the gaps are symmetrically positioned at right angles to the head center line, they will not be centered on their respective tracks when mistracking is present. Either the head, or the medium, or both may be responsible for a mistracking condition. Thus, the head may be laterally displaced from its designated position. On the other hand, the tape may be improperly positioned with respect to the head. This may occur for a number of reasons, one being the improper action of the tape capstans. For example, the capstans may not maintain the tape in the identical lateral position during forward and reverse tape motion respectively, due to skewing of the capstan axis.
When present, mistracking may result in errors of the data read out. For example, a particular core may be reading out data from a track adjacent to the desired one. If mistracking is present during recording, data may be recorded in the wrong track. In both instances errors or the loss of data may result. Even if the head as a whole is tracking properly, data errors and loss of data may occur with respect to one or more tracks of a multi-channel head due to variations in the mutual core spacing beyond predetermined tolerance limits.
Another detrimental effect of mistracking, particularly where magnetic tape heads are concerned, is excessive wear and often premature failure of the head. Under normal operating conditions, the tape edges tend to wear grooves into the head during prolonged use. Accordingly, many leads are provided with a pair of special grooves which bracket the cores of the head between them. The tape edges are in alignment with these grooves when the head is tracking properly. When there is mistracking, the tape edges are out of alignment with the grooves, with one edge moving toward the center of the head. In time, the wear on the head produced by the latter tape edge may cause the head to fail, as well as causing damage to the tape itself. For a normally busy magnetic tape transport, head failure due to uncorrected mistracking may occur in a matter of weeks.
In magnetic tape recording equipment, the head normally abuts a fixed reference surface. Resilient means located on the opposite side of the head urge the head against the reference surface and maintain its lateral position. Appropriate shims, provided between the head and the fixed reference surface, may be varied in order to align the head center line with the tape center line.
In magnetic disk recording equipment mistracking occurs when the head is not centered over the selected circular track. It is corrected by accurately positioning the center of the head in a radial direction over the center of each track and indexing the proper position on the head moving mechanism.
The problem of correcting mistracking lies in the fact that known techniques for doing so are complicated, cumbersome and time consuming and many require a trained specialist to carry them out. In accordance with one of the more accurate head positioning techniques employed, the track (or tracks) are developed by sprinkling the magnetic tape with a fine powder consisting of iron filings or the like. The iron filings adhere to the recorded portion of the track and make it visible to the naked eye. Positioning of the head over the developed track is then carried out by visual observation, usually under a microscope or similar means, to make an accurate determination of the degree of mistracking. Appropriate shims are interposed between the head and the fixed reference surface, as required, until zero displacement is indicated. Because the comparison between the head position and the reference tape must be made under the prevailing lighting conditions which are often less than ideal for the purpose, inaccuracies may occur. Further, since the comparison depends on the experience and the judgment of the observer, the observation is subject to interpretation and therefore it admits of the introduction of errors.
Other known techniques for determining the existence of mistracking rely on an electronic comparison of the position of the magnetic head under observation against a reference tracking medium. A number of these techniques employ frequency modulation to carry out the comparison. Aside from the fact that relatively expensive equipment is required to implement such a technique, the comparison requires a relatively long time interval, e.g. as much as one half the track length of a disk track. Thus, a band of pulses rather than discrete pulses is compared, with a consequent impairment of the achievable sensitivity of the comparison. These techniques are able to determine head mistracking to an accuracy of 1-2 mil, i.e. barely enough to guarantee compatibility of the recorded tape or disk. Additionally, the technique requires the services of a trained technician who can properly interpret the observed results and make the necessary adjustments. Thus, while this method of determining head mistracking is less cumbersome than the method it replaces, its sensitivity is barely adequate and the aforementioned factors combine to make its cost of implementation high.